1. Field of the Invention
The present invention relates to techniques for performing wellsite operations. More particularly, the present invention relates to techniques for determining parameters, such as pressure, of downhole fluids and/or formations.
2. Background of the Related Art
Oilfield operations are typically performed to locate and gather valuable downhole fluids. Typical oilfield operations may include, for example, surveying, drilling, reservoir testing, completions, production, planning, oilfield analysis, fluid injection, fluid storage and abandonment. During such operations, it may be desirable to perform various evaluations (e.g., testing and/or sampling) of downhole parameters. Downhole tools, such as drilling and/or wireline tools, may be provided with devices to perform downhole evaluations of the wellbore and the surrounding formation. Such evaluations may involve the measurement of downhole fluids, such as borehole and/or formation fluids.
Downhole evaluation may require that formation fluid be drawn into the downhole tool for testing and/or sampling. Various fluid communication devices, such as probes, may be extended from the downhole tool to establish fluid communication with the formation and/or surrounding wellbore, and to draw fluid into the downhole tool. A typical probe may extend from the downhole tool and be positioned against the sidewall of the wellbore. A rubber packer at the end of the probe may be used to create a seal with the wellbore wall. Another device used to form a seal with the wellbore wall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings may be used to form a seal with the wellbore wall, and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The downhole tool may draw downhole and/or formation fluids into the downhole tool for testing by one or more sensors within the downhole tool. The sensors may test for various downhole properties, such as pressure and temperature of the downhole fluids. The sensors may be, for example, piezoelectric pressure and temperature transducers. Such transducers may each comprise a crystal resonator located inside a housing structure for the pressure transducer and the temperature transducer. One or more of the sensors may be exposed to borehole fluids for measurement thereof, or isolated therefrom. The sensors may be exposed to harsh conditions, such as extreme temperatures and/or pressures that may affect their quality of measurement.
Electrodes may be placed on opposite sides of each of the resonators (e.g., pressure and temperature) to provide a vibration-exciting field in the resonator. Environmental pressure and temperature may be transmitted to each of the two resonators via the housing and the stresses in the resonator may alter the vibrational characteristics of the resonator. Each of the resonators may be a unitary piezoelectric crystal resonator having a common housing structure in which the resonator is positioned on a median (radial) plane of the cylindrical housing. Crystal end caps may be located at either end of the housing to complete the structure of the transducer. Since the vibration of the resonators may be affected by both temperature and pressure, such devices can be difficult to use in environments where both vary in an uncontrolled manner. Such devices are sometimes referred to as single mode transducers.
Attempts have been made to measure certain downhole parameters as described, for example, in U.S. Pat. Nos. 7,647,979; 7,571,770; 7,568,521; 7,540,165; 7,423,258; 7,363,971; 7,301,223; 7,290,443; 7,268,019; 7,263,880; 7,258,169; 7,246,940; 7,210,344; 7,124,596; 7,117,734; 7,036,579; 7,024,930; 7,017662; 6,877,332; 6,769,296; 6,729,399; 6,672,093; 6,655,458; 6,341,498; 6,147,437; 6,111,340; 5,302,879; 5,265,677; 5,221,873; 4,936,147; 4,802,370; 4,607,530; 4,547,691; 4,407,136; 3,617,780; 2009/0128144; 2009/0045814; 2008/0277162; 2006/0102353; 2006/0101831; 2006/0086506 and in International Patent/Application Nos. WO2006/065559; WO2006/060673; WO2002/037072 and EP552884. In some cases, techniques have been developed for performing downhole evaluations in high temperature and/or hostile environments as described, for example in U.S. Pat. Nos. 7,568,521; 6,336,408; 6,769,487 and in Van Zuilekom and Rourke, “Hostile Formation Testing Advances and Lessons Learned,” Society of Petroleum Engineers (SPE) 124048, SPE Annual Technical Conference and Exhibition held in New Orleans, La., USA, 4-7 Oct. 2009.
Despite the development of techniques for measuring downhole parameters, there remains a need to provide advanced techniques for determining parameters of downhole formations and/or wellbore fluids. The present invention is directed at fulfilling such need.